1. Field of the Invention
This invention relates to a photocurable and thermosetting resin composition developable with an aqueous alkali solution which is useful as a resist ink for use in the manufacture of a printed circuit board, and more particularly as a photographically developable ink capable of forming an image through exposure to an active energy ray and subsequent development with an aqueous alkali solution and giving rise to a cured film exhibiting flexibility and resistance to the heat of soldering and which is particularly useful as a solder resist ink to be applied to a flexible printed circuit board.
2. Description of the Prior Art
To protect a wiring (circuit) pattern formed on a substrate as by the screen printing technique against the action of an external environment or to preclude accidental adhesion of solder to the areas abhorring solder in the soldering process which is carried out in mounting electronic parts on the surface of a printed circuit board, the practice of depositing on the printed circuit board a protective layer called a cover coat or solder mask is prevalent.
The solder resist inks which have heretofore found utility in the field of applications mentioned above are mainly based on polyfunctional epoxy resin-containing compositions. The cured films which are produced with these solder resist inks indeed exhibit satisfactory resistance to heat but suffer from deficiency in flexibility. These solder resist inks, therefore, have had their utility limited to rigid boards which are not demanded to exhibit the flexibility (folding property) that is proper for a cured film. Thus, they do not find easy acceptance for use in flexible printed circuit boards (FPC) which have been finding growing use in recent years.
In the light of the true state of affairs mentioned above, numerous species of resist inks whose cured films possess flexibility have been proposed. Published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as "JP-A-") 2-269,166, for example, proposes a thermosetting solder resist ink which comprises polyparabanic acid, epoxy resin, and a polar solvent and JP-A-6-41,485 proposes a thermally drying solder resist ink which has polyparabanic acid and phenoxy resin as essential components thereof. Since these solder resists are intended to form a resist pattern by means of screen printing, however, they are at a disadvantage in producing such phenomena as exudation from paper or inferior transfer to a surface during the course of printing, limiting the allowable width of lines of a screen, and incurring difficulty in allowing formation of an image so fine as to conform to the latest level of densification of circuits.
In more recent years, therefore, solder resist inks of the photographic developing type have been proposed as by, for example, JP-A-2-173,749, JP-A-2-173,750, and JP-A-2-173,751. They are, however, incapable of providing fully ample flexibility of cured film aimed at.